Functional membrane properties of chick heart change dramatically between the time the organ begins beating (36 hrs) and the period when it has established its definitive 4-chambered structure (7 days). We will analyze two specific membrane systems that differentiate during this period: the tetrodotoxin-sensitive fast sodium channel, and the insulin-sensitive hexose transport mechanism. The development of both systems will be investigated in embryonic chick heart cells under a variety of different culture conditions, for which a body of background information already exists, and in embryonic mouse heart cells for comparison. Hearts will be dissociated into their component cells and cultured as spontaneously beating isolated singlets, monolayer sheets or spheroidal aggregates. The appearance of TTX-sensitive Na-specific channels will be assessed by current-voltage curves and voltage-clamp analysis of preparations taken from different aged embryos. Similar analyses will be performed on cells exposed to current-blocking agents, inhibitors of protein synthesis, and proteolytic enzymes. Tritium-labeled TTX and BTX will be used to determine the stoichiometry of binding to single cells, aggregates, or intact heart tissue. Similar heart-cell preparations will be used to investigate the developmental control mechanisms underlying a hexose transport system, its differentiation and the molecular mechanisms that regulate the appearance of both an insulin-responsive and non-responsive system. The kinetics of uptake and efflux of labeled hexose or amino acids will be studied in cells from different aged hearts, subjected to variations in substrate concentration, ion content, pH, temperature, competitive and non-competitive inhibitors, and inhibitors of transciption and translation.